2013年12月

Linux 2.6.25中的select系统调用
主要有4个函数:
sys_select:处理时间参数,调用core_sys_select。
core_sys_select:处理三个fd_set参数,调用do_select。
do_select:做select/poll的工作。在合适的时机把自己挂起等待,调用sock_poll。
sock_poll:用函数指针分派到具体的协议层函数tcp_poll、udp_poll、datagram_poll。
层层分工明确,我也要多学习这种方式啊。

/*
sys_select(fs/select.c)
处理了超时值(如果有),将struct timeval转换成了时钟周期数,调用core_sys_select,然后检查剩余时间,处理时间
*/
asmlinkage long sys_select(int n, fd_set __user *inp, fd_set __user *outp,
						   fd_set __user *exp, struct timeval __user *tvp)
{
	s64 timeout = -1;
	struct timeval tv;
	int ret;
	if (tvp) {/*如果有超时值*/
		if (copy_from_user(&tv, tvp, sizeof(tv)))
			return -EFAULT;
		if (tv.tv_sec < 0 || tv.tv_usec < 0)/*时间无效*/
			return -EINVAL;
		/* Cast to u64 to make GCC stop complaining */
		if ((u64)tv.tv_sec >= (u64)MAX_INT64_SECONDS)
			timeout = -1;	/* 无限等待*/
		else {
			timeout = DIV_ROUND_UP(tv.tv_usec, USEC_PER_SEC/HZ);
			timeout += tv.tv_sec * HZ;/*计算出超时的相对时间,单位为时钟周期数*/
		}
	}
	/*主要工作都在core_sys_select中做了*/
	ret = core_sys_select(n, inp, outp, exp, &timeout);
	if (tvp) {/*如果有超时值*/
		struct timeval rtv;
		if (current->personality & STICKY_TIMEOUTS)/*模拟bug的一个机制,不详细描述*/
			goto sticky;
		/*rtv中是剩余的时间*/
		rtv.tv_usec = jiffies_to_usecs(do_div((*(u64*)&timeout), HZ));
		rtv.tv_sec = timeout;
		if (timeval_compare(&rtv, &tv) >= 0)/*如果core_sys_select超时返回,更新时间*/
			rtv = tv;
		/*拷贝更新后的时间到用户空间*/
		if (copy_to_user(tvp, &rtv, sizeof(rtv))) {
sticky:
			/*
			* If an application puts its timeval in read-only
			* memory, we don't want the Linux-specific update to
			* the timeval to cause a fault after the select has
			* completed successfully. However, because we're not
			* updating the timeval, we can't restart the system
			* call.
			*/
			if (ret == -ERESTARTNOHAND)/*ERESTARTNOHAND表明,被中断的系统调用*/
				ret = -EINTR;
		}
	}
	return ret;
}
/*core_sys_select
为do_select准备好了位图,然后调用do_select,将返回的结果集,返回到用户空间
*/
static int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
						   fd_set __user *exp, s64 *timeout)
{
	fd_set_bits fds;
	void *bits;
	int ret, max_fds;
	unsigned int size;
	struct fdtable *fdt;
	/* Allocate small arguments on the stack to save memory and be faster */
	/*SELECT_STACK_ALLOC 定义为256*/
	long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
	ret = -EINVAL;
	if (n < 0)
		goto out_nofds;
	/* max_fds can increase, so grab it once to avoid race */
	rcu_read_lock();
	fdt = files_fdtable(current->files);/*获取当前进程的文件描述符表*/
	max_fds = fdt->max_fds;
	rcu_read_unlock();
	if (n > max_fds)/*修正用户传入的第一个参数:fd_set中文件描述符的最大值*/
		n = max_fds;
	/*
	* We need 6 bitmaps (in/out/ex for both incoming and outgoing),
	* since we used fdset we need to allocate memory in units of
	* long-words.
	*/
	/*
	如果stack_fds数组的大小不能容纳下所有的fd_set,就用kmalloc重新分配一个大数组。
	然后将位图平均分成份,并初始化fds结构
	*/
	size = FDS_BYTES(n);
	bits = stack_fds;
	if (size > sizeof(stack_fds) / 6) {
		/* Not enough space in on-stack array; must use kmalloc */
		ret = -ENOMEM;
		bits = kmalloc(6 * size, GFP_KERNEL);
		if (!bits)
			goto out_nofds;
	}
	fds.in      = bits;
	fds.out     = bits +   size;
	fds.ex      = bits + 2*size;
	fds.res_in  = bits + 3*size;
	fds.res_out = bits + 4*size;
	fds.res_ex  = bits + 5*size;
	/*get_fd_set仅仅调用copy_from_user从用户空间拷贝了fd_set*/
	if ((ret = get_fd_set(n, inp, fds.in)) ||
		(ret = get_fd_set(n, outp, fds.out)) ||
		(ret = get_fd_set(n, exp, fds.ex)))
		goto out;
	zero_fd_set(n, fds.res_in);
	zero_fd_set(n, fds.res_out);
	zero_fd_set(n, fds.res_ex);
	/*
	接力棒传给了do_select
	*/
	ret = do_select(n, &fds, timeout);
	if (ret < 0)
		goto out;
	/*do_select返回,是一种异常状态*/
	if (!ret) {
		/*记得上面的sys_select不?将ERESTARTNOHAND转换成了EINTR并返回。EINTR表明系统调用被中断*/
		ret = -ERESTARTNOHAND;
		if (signal_pending(current))/*当当前进程有信号要处理时,signal_pending返回真,这符合了EINTR的语义*/
			goto out;
		ret = 0;
	}
	/*把结果集,拷贝回用户空间*/
	if (set_fd_set(n, inp, fds.res_in) ||
		set_fd_set(n, outp, fds.res_out) ||
		set_fd_set(n, exp, fds.res_ex))
		ret = -EFAULT;
out:
	if (bits != stack_fds)
		kfree(bits);/*对应上面的kmalloc*/
out_nofds:
	return ret;
}
/*do_select
真正的select在此,遍历了所有的fd,调用对应的xxx_poll函数
*/
int do_select(int n, fd_set_bits *fds, s64 *timeout)
{
	struct poll_wqueues table;
	poll_table *wait;
	int retval, i;
	rcu_read_lock();
	/*根据已经打开fd的位图检查用户打开的fd, 要求对应fd必须打开, 并且返回最大的fd*/
	retval = max_select_fd(n, fds);
	rcu_read_unlock();
	if (retval < 0)
		return retval;
	n = retval;
	/*将当前进程放入自已的等待队列table, 并将该等待队列加入到该测试表wait*/
	poll_initwait(&table);
	wait = &table.pt;
	if (!*timeout)
		wait = NULL;
	retval = 0;
	for (;;) {/*死循环*/
		unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;
		long __timeout;
		/*注意:可中断的睡眠状态*/
		set_current_state(TASK_INTERRUPTIBLE);
		inp = fds->in; outp = fds->out; exp = fds->ex;
		rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;
		for (i = 0; i < n; ++rinp, ++routp, ++rexp) {/*遍历所有fd*/
			unsigned long in, out, ex, all_bits, bit = 1, mask, j;
			unsigned long res_in = 0, res_out = 0, res_ex = 0;
			const struct file_operations *f_op = NULL;
			struct file *file = NULL;
			in = *inp++; out = *outp++; ex = *exp++;
			all_bits = in | out | ex;
			if (all_bits == 0) {
				/*
				__NFDBITS定义为(8 * sizeof(unsigned long)),即long的位数。
				因为一个long代表了__NFDBITS位,所以跳到下一个位图i要增加__NFDBITS
				*/
				i += __NFDBITS;
				continue;
			}
			for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) {
				int fput_needed;
				if (i >= n)
					break;
				/*测试每一位*/
				if (!(bit & all_bits))
					continue;
				/*得到file结构指针,并增加引用计数字段f_count*/
				file = fget_light(i, &fput_needed);
				if (file) {
					f_op = file->f_op;
					mask = DEFAULT_POLLMASK;
					/*对于socket描述符,f_op->poll对应的函数是sock_poll
					注意第三个参数是等待队列,在poll成功后会将本进程唤醒执行*/
					if (f_op && f_op->poll)
						mask = (*f_op->poll)(file, retval ? NULL : wait);
					/*释放file结构指针,实际就是减小他的一个引用计数字段f_count*/
					fput_light(file, fput_needed);
					/*根据poll的结果设置状态,要返回select出来的fd数目,所以retval++。
					注意:retval是in out ex三个集合的总和*/
					if ((mask & POLLIN_SET) && (in & bit)) {
						res_in |= bit;
						retval++;
					}
					if ((mask & POLLOUT_SET) && (out & bit)) {
						res_out |= bit;
						retval++;
					}
					if ((mask & POLLEX_SET) && (ex & bit)) {
						res_ex |= bit;
						retval++;
					}
				}
				/*
				注意前面的set_current_state(TASK_INTERRUPTIBLE);
				因为已经进入TASK_INTERRUPTIBLE状态,所以cond_resched回调度其他进程来运行,
				这里的目的纯粹是为了增加一个抢占点。被抢占后,由等待队列机制唤醒。
				在支持抢占式调度的内核中(定义了CONFIG_PREEMPT),cond_resched是空操作
				*/
				cond_resched();
			}
			/*根据poll的结果写回到输出位图里*/
			if (res_in)
				*rinp = res_in;
			if (res_out)
				*routp = res_out;
			if (res_ex)
				*rexp = res_ex;
		}
		wait = NULL;
		if (retval || !*timeout || signal_pending(current))/*signal_pending前面说过了*/
			break;
		if(table.error) {
			retval = table.error;
			break;
		}
		if (*timeout < 0) {
			/*无限等待*/
			__timeout = MAX_SCHEDULE_TIMEOUT;
		} else if (unlikely(*timeout >= (s64)MAX_SCHEDULE_TIMEOUT - 1)) {
			/* 时间超过MAX_SCHEDULE_TIMEOUT,即schedule_timeout允许的最大值,用一个循环来不断减少超时值*/
			__timeout = MAX_SCHEDULE_TIMEOUT - 1;
			*timeout -= __timeout;
		} else {
			/*等待一段时间*/
			__timeout = *timeout;
			*timeout = 0;
		}
		/*TASK_INTERRUPTIBLE状态下,调用schedule_timeout的进程会在收到信号后重新得到调度的机会,
		即schedule_timeout返回,并返回剩余的时钟周期数
		*/
		__timeout = schedule_timeout(__timeout);
		if (*timeout >= 0)
			*timeout += __timeout;
	}
	/*设置为运行状态*/
	__set_current_state(TASK_RUNNING);
	/*清理等待队列*/
	poll_freewait(&table);
	return retval;
}
static unsigned int sock_poll(struct file *file, poll_table *wait)
{
	struct socket *sock;
	/*约定socket的file->private_data字段放着对应的socket结构指针*/
	sock = file->private_data;
	/*对应了三个协议的函数tcp_poll,udp_poll,datagram_poll,其中udp_poll几乎直接调用了datagram_poll
	累了,先休息一下,这三个函数以后分析*/
	return sock->ops->poll(file, sock, wait);
}

其他重要函数一览
static int max_select_fd(unsigned long n, fd_set_bits *fds)
返回在fd_set中已经打开的,并且小于用户指定最大值,的fd
static inline int get_fd_set(unsigned long nr, void __user *ufdset, unsigned long *fdset)
从用户空间拷贝fd_set到内核
static inline void zero_fd_set(unsigned long nr, unsigned long *fdset)
把fd_set清零
static inline unsigned long __must_check set_fd_set(unsigned long nr, void __user *ufdset, unsigned long *fdset)
把fd_set拷贝回用户空间
static inline int signal_pending(struct task_struct *p)
目前进程有信号需要处理
struct file *fget_light(unsigned int fd, int *fput_needed)
由fd得到其对应的file结构指针,并增加其引用计数
static inline void fput_light(struct file *file, int fput_needed)
释放由fget_light得到的file结构指针,减少其引用计数
set_current_state
设置当前进程的状态
static inline int cond_resched(void)
判断是否有进程需要抢占当前进程,如果是将立即发生调度。就是额外增加一个抢占点。
signed long __sched schedule_timeout(signed long timeout)
当前进程睡眠timeout个jiffies
rcu_read_lock
rcu_read_unlock
Linux 2.6新加入的rcu锁。读锁的加锁、解锁函数
参考http://www.ibm.com/developerworks/cn/linux/l-rcu
poll_freewait
poll_initwait
poll_wait
...
和文件IO,poll机制有关的几个函数,参考《Linux设备驱动(第三版)》6.3
tcp_poll
udp_poll
datagram_poll
协议层的poll函数
文件出处:http://zhangyafeikimi.iteye.com/blog/248815